LOS ANGELES, California (CNN) -- A routine maintenance check wasn't performed on an air traffic control radio system serving much of the West, causing the system to shut down for five hours Tuesday, the FAA reported Wednesday.

The FAA has launched an investigation into why the maintenance wasn't performed.

Hundreds of planes were in the air at the time. Travel to and from multiple airports, primarily Los Angeles and San Diego, California, was disrupted.

The FAA said several planes came dangerously close.

Under FAA guidelines, planes are to be five miles apart horizontally and 2,000 feet vertically.

Pilots reported no near-misses, the agency said.

"The FAA is aggressively investigating Tuesday evening's radio communication outage," the FAA said in a statement. "Although FAA air traffic control systems have nearly perfect reliability, any system failure -- no matter how rare -- is unacceptable."

The FAA said radio contact was lost because routine maintenance was not performed on the primary and radio and voice communications with planes. When a check is not performed, it results in the system turning off.

A backup system that is to take over in such situations did not turn on because it was improperly configured.

"Our preliminary findings indicate that the outage was not the result of system reliability, but rather an event that should've been avoided," the FAA said.

The outage resulted in hundreds of flights being canceled, delayed or diverted, with the FAA imposing a ground stop for several hours at multiple airports, including Los Angeles International.

The center where the outage occurred controls air traffic for California, Arizona, Nevada and parts of Utah.

The FAA emphasized that only radio communications were lost to the planes and that radar coverage remained fully operational during the five-hour outage.